1. Field of the Invention
The present invention relates to economical collection and transmission of data via wireless data telemetry utilizing a plurality of transceivers, each preferably configured with single antenna.
2. Discussion of the Prior Art
Infrared and radio frequency (RF) data transmission methods are the principal wireless communication technologies described in the prior art. Infrared beam communications systems cannot operate over distances of more than a few feet and so are limited to applications such as bar code scanning and television (or other home appliance) remote control.
As a result, most of the prior art wireless data transmission products utilize standard RF technology, i.e., radios, the same technology used in vehicle dispatch and police communication systems. Standard RF products are relatively simple and inexpensive to build, but for operation FCC licenses may be required. RF transmissions are susceptible to interference from a growing number of sources and to interception by readily available eavesdropping equipment. The unreliable quality of standard RF transmissions makes the technology unsuitable for applications where all of the information transmitted must be accurate, complete, and secure.
In order to overcome the shortcomings of standard RF transmission methods, direct sequence spread spectrum (DSSS) was developed. DSSS radios divide or slice transmissions into small bits, thereby spreading energy from the bits simultaneously across a wide spectrum of radio frequencies. DSSS is a relatively unreliable transmission medium, however, because spreading the message across a wide spectrum greatly reduces the strength of the radio signal carrying the message on any one frequency. Since a DSSS receiver must simultaneously monitor the entire allotted spectrum, severe interference from a high energy RF source within the monitored spectrum can pose an insurmountable problem. DSSS performance also degrades quickly in shared-service environments having multiple radio systems operating simultaneously.
Frequency hopping spread spectrum (FHSS) technology was developed by the U.S. military to prevent interference with or interception of radio transmissions on the battle field and is employed by the military in situations where reliability and speed are critical. Standard RF and DSSS cannot match the reliability and security provided by frequency hopping. Instead of spreading (and therefore diluting) the signal carrying each bit across an allotted spectrum, as in DSSS, frequency hopping radios concentrate full power into a very narrow spectral width and randomly hop from one frequency to another in a sequence within a defined band, up to several hundred times per second. Each FHSS transmitter and receiver coordinate the hopping sequence by means of an algorithm exchanged and updated by both transmitter and receiver on every hop. Upon encountering interference on a particular frequency, the transmitter and receiver retain the affected data, randomly hop to another point in the spectrum and then continue the transmission. There should always be frequencies somewhere in the spectrum that are free of interference, since neither benign producers of interference or hostile jammers will likely interfere with all frequencies simultaneously and at high power radiation levels, and so the frequency hopping transmitter and receiver will find frequencies with no interference and complete the transmission. This ability to avoid interference enables FHSS radios to perform more reliably over longer ranges than standard RF or DSSS radios. In the prior art, frequency hopping FHSS communication systems have been used almost exclusively in the extremely expensive robust military or government communication systems.
Generally speaking, data telemetry is the transmission of short packets of information from equipment or sensors to a recorder or central control unit. The data packets are transferred as electric signals via wire, infrared or RF technologies and data is received at a central control unit such as a computer with software for automatically polling and controlling the remote devices. The control unit analyzes, aggregates, archives and distributes the collected data packets to other locations, as desired, via a local area network (LAN) and/or a wide area network (WAN). Wireless data telemetry provides several advantages over data telemetry on wired networks. First, wireless systems are easier and less expensive to install; second, maintenance costs are lower; third, operations can be reconfigured or relocated very quickly without consideration for rerunning wires, and fourth, wireless telemetry offers improved mobility during use.
The Federal Communications Commission (FCC) has designated three license-free bandwidth segments of the radio frequency spectrum and made them available for industrial, scientific and medical (ISM) use in the United States. These three segments are 900 MHZ, 2.4 GHz and 5.8 GHz. Anyone may operate a wireless network in a license-free band without site licenses or carrier fees and is subject only to a radiated power restriction (i.e., a maximum of one watt radiated power). The radio signals transmitted must be spread spectrum. Foreign national spectrum regulation organizations and international telecommunications bodies have also agreed to recognize a common license-free ISM frequency at 2.4 GHz, and so a defacto international standard for license-free ISM communications has emerged. The ISM band at 2.4 GHz provides more than twice the bandwidth capacity and is subject to far less congestion and interference than the ISM band at 900 MHZ. Several industrial nations do not permit a license-free ISM band at 900 MHZ and relatively few nations have a license-free ISM band at 5.8 Ghz, but the United States, Europe, Latin America and many Asian countries have adopted an ISM band at 2.4 GHz.
Not just any wireless telemetry system will do for many applications, however. The realities of the marketplace dictate that data telemetry cannot be the most expensive part of a system having commercial application. For example, if a retail point-of-sale cash register is to be configured with a wireless data telemetry radio; the radio cannot be more expensive than the cash register. In many commercial applications, buyers have fixed expectations for what things cost and new features, however useful, cannot substantially exceed those expectations. Thus, it would be best if the wireless data telemetry radio were free. In the interest of providing the most economical wireless data telemetry radio, a transceiver with a shared antenna for both transmit and receive segments is suggested, but how is the switching between transmit function and receive function to be accomplished? The off-the-shelf transmit/receive (T/R) switches are expensive, have a high parts count, and are often configured such that the components within the switch dissipate transmitter energy when in the receive state, adding heat and raising the energy required to operate the wireless data telemetry transceiver. Use of off-the-shelf T/R switch components, as is customary in the prior art, make it difficult to provide a wireless data telemetry radio that is small, light, resistant to interference from adjacent RF noise sources, and uses as little energy as possible.
What is needed, then, is an inexpensive, easy to use and robust data telemetry and communication system including an inexpensive and compact transceiver, preferably operating in the common license-free ISM frequency band, and providing reliable communications for a variety of users in commercial and industrial environments.
Accordingly, it is a primary object of the present invention to overcome the above mentioned difficulties by providing an economical, compact wireless data telemetry transceiver is adapted to establish and maintain communication links in the license-free ISM frequency band at 2.4 GHz.
Another object of the present invention is to efficiently switch between the transmit and receive functions in a transceiver, while dissipating as little transmitter power as possible when in the receive state.
Yet another object of the present invention is to implement an economical and reliable transmit/receive switch with the smallest possible parts count.
Still another object of the present invention is to implement a transmit/receive switch with inexpensive parts.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, an economical, compact wireless data telemetry transceiver is adapted to establish and maintain communication links at 2.4 GHz in the license-free ISM frequency band and preferably provides a good balance between data rate and range, for example, providing 9.6 kilobits per second (9.6 Kbps) data transmission over an outdoor line of sight range of approximately 35 thousand meters.
The communication system of the present invention includes components ideally suited to specific wireless data telemetry applications. A transceiver is configured as a printed circuit card having an edge connector. The wireless transceiver includes RF and computer control components in a compact package approximately the size of a deck of cards and is adapted to be built into original equipment manufacturer (OEM) products to support a wide range of wireless data telemetry applications. Each transceiver includes a shielded RF board or module with a frequency hopping transmitter and receiver, an antenna, and a digital control board or module. The digital control module performs RF module and application interface management and an application interface is included to communicate with specific OEM products utilizing serial transistor/transistor logic (TTL) or other standard interfaces. The transceiver operates in the license-free portion of the FCC designated ISM frequency band at 2.4 GHz; as noted above, one embodiment of the transceiver transmits and receives data at 9.6 Kbps at ranges of up to 1500 feet when used indoors with the integrally housed antenna, or up to 12 miles line of sight when used outdoors with an optional directional antenna. The transceiver transmits or receives on any of 550 independent, non-interfering frequencies. When using the transceiver, a data telemetry network can readily be configured for either point-to-point (e.g. wire replacement) or host-to-multipoint networks linked to a user""s existing computer or to telephone networks via a system gateway. Optionally, up to 5 collocated independent networks may operate simultaneously, and data security is provided by rapid and random frequency changes (i.e., frequency hopping); the transceiver can optionally be used with data encryption software for providing secure, coded transmissions.
Alternatively, a connector transceiver can be attached to a computer or other device using a standard serial (RS232) port. The connector duplicates the functions of the transceiver but is housed in an enclosure having a cord terminated with an RS232 compatible connector. The connector can therefore be used with a wide variety of existing products such as cash registers, ATM machines, laptop computers or any other computer controlled device having an RS232 port and capable of utilizing the frequency hopping spread spectrum communication system software described in the attached appendices.
A plurality of optional antennas can be used with either the transceiver or the long range connector. The standard antenna included with either the long range connector or the long range transceiver is an omni-directional antenna having vertical polarization and a spherical radiation pattern, is built into the transceiver or connector housings and does not require an added cable.
The transceiver functions as a half duplex, bi-directional communication device; preferably, transmit and receive functions are time interleaved in a non-overlapping fashion, consistent with the requirements of a frequency hopping radio. The transmit interval is restricted to less than 0.4 seconds. In the course of a normal information exchange, a given transmission is generated on a frequency selected from a set of all available hop frequencies. The transmission is limited in duration to the availability of incoming data, and following the transmission, the radio switches to a receive mode and processes any incoming data. Once reception is complete, the transmit interval/receive interval cycle is restarted on a new frequency selected from the hop frequency set. Transmit receive cycling continues until all 75 unique frequencies in the set have been used, whereupon the frequency selection process reenters the top of the table and begins reusing the same 75 frequencies.
Transmitted data is directly modulated onto a synthesized carrier by use of minimum shift keying (MSK) modulation. The receiver is a dual conversion super heterodyne, down converting the received signal first to a 315 MHZ intermediate frequency (IF) signal and then down converting a second time to a 10.7 MHZ IF signal. Demodulation is accomplished using a limiter/discriminator circuit and the demodulated data is recovered from the demodulator output by processing through a comparator. First and second local oscillators (LOs) are controlled in frequency by use of a single loop indirect frequency synthesis. Samples of both first and second voltage controlled oscillators (VCOs) are divided down using phase-locked loop integrated circuit elements, where each sample is compared to an onboard 8 MHZ crystal reference oscillator. During the transmit interval, a single transmitter VCO is controlled by the same device and in the same manner.
To minimize total power consumption within the transceiver, portions of circuitry not in use during either the transmit or receive intervals are disabled under control of the system controller.
The RF Board consists of a transmitter, receiver, frequency synthesizer and T/R Switch. Each of these sections is controlled by an external microprocessor to either transmit serial data or receive serial data. The basic transmitted signal is generated by a voltage-controlled-oscillator (VCO) that operates in the 2.4 to 2.4835 GHz frequency band. The signal is then amplified by three stages of amplification. All three amplification stages and the VCO are switched ON for transmit and switched OFF for receive. A power amplifier stage provides 26 dBm of output power to drive the antenna. This stage also uses a GaAs RF Power FET and a similar power control circuit. The transmitted signal passes through the T/R switch and a 2.44 GHz 4-pole bandpass filter to the antenna. Both the T/R switch and the bandpass filter are implemented using strip line on a separate daughter board.
The receiver uses dual conversion with a first IF of 315 MHZ and a second IF of 10.7 MHZ. The received signal from the antenna passes through the same 2.44 GHz filter the transmitted signal passed through and then passes through the T/R switch to the LNA.
The analog serial data stream is digitized by thresholding the signal using a comparator and a threshold generated from a peak follower. The peak follower follows both the positive and negative peaks of the analog serial data stream and then generates a threshold signal that is half way between the two peaks. The output of the comparator is the digital received signal output to the digital board.
The RF Board includes an I/O Interface which consists of two mechanical connections. Most of the connections are made via a 20 pin dual in-line header. The other connection is for the antenna and is a microstrip pad and ground connection to which the coaxial antenna cable is soldered. TTL-compatible input signals on the Rx/Tx-pin are used to control the T/R switch. A logic high on this pin puts the T/R switch in the receive position and a logic low puts it in the transmit position. Before the radio switches from receive mode (Rx) to transmit mode (Tx), the T/R switch should be put in the Tx position. When switching from Tx to Rx the T/R switch should remain in the Tx position until after the radio is switched from Tx to Rx.
The RF Board includes an RF I/O connection. When data is presented to the serial port of the digital board, firmware on the digital board will cause the radio to hop on 75 frequencies in the 2400-2483.5 MHz band. The dwell time for each hop is 31.6 ms. During a single hop the carrier is frequency modulated with the transmit serial data stream from the digital board. Immediately after the transmit time period the radio switches to the receive mode.
The novel transmit/receive (T/R) switch of the present invention is located on the RF board; preferably, two P-Intrinsic-N (PIN) diodes are used to implement the switching functions. Neither of the PIN diodes is in the transmit power path, so the raw transmitter power doesn""t actually flow through either PIN diode, and when no energy is applied to either diode, the T/R switch is correctly applied to one of its two states or positions, and so, in effect, the T/R switch is a single pull, double throw implementation of a classic transmit/receive switch, connected with the antenna. In the first state, the PIN diodes are turned on, the transmitter is connected to the antenna port and the receive port is short circuited, but connected via a quarter wavelength long transmission line segment, so electrically, it appears the receive port is an open circuit. In the second state, the PIN diodes are turned off, the receiver is connected to the antenna port and the transmitter port is effectively short circuited.
The T/R switch is fabricated using electrically controlled transmission line lengths (e.g., in either a strip line or microstrip matrix) that are controlled in electrical length by selective shorting or grounding the transmission line matrix at selected points along the transmission line segments.
The T/R switch is also configurable as separable xe2x80x9cRF outxe2x80x9d and xe2x80x9cRF inxe2x80x9d components which can be individually actuated via a selectively controllable bias line. Multiple xe2x80x9cRF outxe2x80x9d and xe2x80x9cRF inxe2x80x9d components can be configured into a Multiport RF hub connectable to an antenna or another common port, to permit selection of energy flow direction out of or into selected xe2x80x9cRF outxe2x80x9d or xe2x80x9cRF inxe2x80x9d ports.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings, wherein like reference numerals in the various figures are utilized to designate like components.